1. Field of the Invention
The present invention relates to an engine valve of titanium (Ti) based alloy which is suitably used in various internal combustion engines for automobiles or the like.
2. Prior Art
An engine valve of Ti based alloy has been recently developed for use in various internal combustion engines for automobiles and the like in order to obtain light weight engines, and has been put to partial practical use.
Among the conventional Ti alloy engine valves, an intake valve has been manufactured using a Ti alloy having a representative composition of Ti-6%Al(aluminum)-4%V(vanadium) by weight, while an exhaust valve has been made of a Ti alloy having a representative composition of Ti-6%Al-2%Sn(tin)-4%Zr(zirconium)-2%Mo(molybdenum)-0.1%Si(silicon). For manufacturing the engine valve, an ingot of the above alloy has been first subjected to hot working such as hot forging and hot rolling, to thereby produce a bar stock (wire member) of a prescribed length, and then a head portion has been formed at one end thereof by means of hot upset forging.
In the conventional Ti alloy engine valves, however, since Ti alloy is less suited to hot working, repeated working operations with small degrees of working have been required, so that the processing cost has been unduly increased. In addition, inasmuch as the selection of the kind of Ti alloy has been restricted due to the difficulty in workability. Therefore, Ti alloy having desired properties cannot be utilized, so that Ti alloy engine valves having satisfactory characteristics cannot be successfully obtained.